The invention relates generally to wireless communications. More specifically, the invention relates to a wireless basestation and/or a user terminal that uses downlink diversity.
The demand for wireless communications services greatly outstrips the current and projected future supply, especially on the downlink (i.e., the link between the basestation and the mobile terminal, also referred to as the forward link). To meet this demand, novel technologies that improve the link level performance of wireless systems can be employed. In particular, the performance degradation due to multiple access interference (MAI) and multipath fading should be overcome. Known downlink methodologies fail to provide universal performance enhancing solutions because they either rely on multiple antennas at the mobile terminal, work effectively only in parts of the coverage area, or rely on specific characteristics of the propagation environment that are not always present in wireless communications systems.
In pursuit of a universally effective downlink solution, the most promising known approaches combine downlink diversity (DD) with spatial division multiple access (SDMA). While DD ensures robustness against signal fading, SDMA greatly reduces the effects of multiple access interference.
Known empirical evidence shows the effectiveness of DD in reducing the probability of signal outage due to deep fades. While diversity antennas at the user terminal can serve a similar purpose, these antenna are often impractical due to the required increase in terminal cost and size. See, e.g., Golden, G. D., et. al., “Detection algorithm and initial laboratory results using V-BLAST space-time communication architecture,” Electr. Lett., vol. 35(1), pp. 14–16, Jan. 1999. Therefore, most known DD methods employ multiple transmissions from one or more basestation(s). The transmitted signals are diversity coded to ensure they can either be separated or added coherently at the user terminal. Diversity coding may be implemented in various ways and can be done with or without feedback from the user terminal as described, for example, in Dabak et al., “A comparison of the open loop transmit diversity schemes for third generation wireless systems”, Proceedings of the 2000 IEEE Wireless Communications and Networking Conference, 2000, vol. 1, pp. 437–442.
While DD combats fading, SDMA greatly reduces MAI because each signal transmission, and thus the interference caused to other users in the system, is confined to just a portion of the cell area. One widely deployed SDMA technique is sectorization, where the entire cell is split into three or more sectors, each of which is treated as a separate cell in the sense that signals are sent within that sector without being sent into other sectors. A more effective SDMA technique is beamforming, which uses one or more beams to serve a user within a sector. The beams can be chosen from a predefined set, as is done with so-called multi-beam antennas, or can be formed adaptively according to some optimization criterion.
Recently, various methods have been proposed that combine DD and SDMA. For example, some of these known methodologies use a single array and spatially orthogonal beams for the downlink transmission. As a consequence, “the downlink performance is dictated by the angular spread of the radio environment” and “best results were (are) found for large angular spread”, as concluded by Katz et al. in “Extension of space-time coding to beamforming WCDMA basestations”, Proceedings of the 51st IEEE Vehicular Technology Conference, May 2000, vol. 2, pp. 1230–1234.
At least one known system does not require spatially orthogonal beams. U.S. Pat. No. 6,201,801, entitled “Polarization diversity phased array cellular basestation and associated methods” discloses a system that uses two antenna arrays transmitting successive signal segments (e.g. TDMA frames) using alternatingly one then the other antenna array having different polarizations. Although in this system the fading of the signal received at the mobile station can be made substantially uncorrelated in successive segments at any given time, the user terminal can only receive a single multipath component. Consequently, the benefit of diversity is not obtained.
Other known systems combine SDMA and DD for terminals located in some regions by simultaneously transmitting from antennas located at spatially distinct basestations, an approach known as handover. While this approach depends neither on the spatial nor on the temporal properties of the propagation environment, it is only effective if the user terminal is located in a suitable area that is simultaneously covered by at least two basestations.
The performance gain of these various known systems is disadvantageously dependent on the location of the user terminal, and on the spatial and temporal properties of the radio environment. Thus, a need exists for combining DD and SDMA more effectively to thereby overcome this dependency, while also substantially reducing MAI.